The power IC is widely applied for power management and power control. By controlling the power switching device to switch on or off, the power IC can perform the function of power management and power control. The power MOS (metal/oxide/semiconductor) is the most popular and widely used power device currently.
The primary characteristics of the power switching devices include of low manufacturing cost, low resistance in conducting state and fast switching. The low resistance is benefit for very low voltage drop under high driving current. The trench power MOS transistor with above property can refer to the U.S. Pat. No. 6,974,750. As shown in FIG. 1, the plural trenches are formed in an n− epitaxial layer 5b on a heavy doped n+ semiconductor substrate 1. A trench oxide layer 8 is formed on all surfaces in the trenches 7 (the sidewalls, bottoms and mesas). Then, an n type doped polysilicon layer 9 is formed on the trench oxide layer 8 to serve as a gate. The p− body 4b and the n+ implanted areas 6 as a source are formed in the n− epitaxial layer 5b below the mesas between trenches. An oxide layer 10 is formed on the polysilicon layer 9. The oxide layer 10 is then etched by photolithography to form recess thereon. The etching is performed even through the n+ implanted area 6 until the p− body 4b to form a “V” shape source contact area. Under the source contact area is a p+ heavy doped area 12 formed by performing an ion implanting procedure. A metal layer 13 is formed to fill in the “V” shape source contact area. The drain metal layer 14 is formed on the rear surface of the heavy doped n+ semiconductor substrate 1.